Bulletin No. 19 


Mineral Technology Series No. 10 


University of Arizona 
Bulletin 

CHARLES F. WILLIS, Director 


Gypsum 

By Frank L. Culin, Jr. 

•i 



PUBLISHED BY THE 

University of Arizona 
Bureau of Mines 

TUCSON, ARIZONA 

1915-16 


Monograph 





BIBLIOGRAPHY 


Adams, G. I. and others. Gypsum deposits of the United States. 

U. S. G. S. Bull. 223. 123 pp. 1904. 

Boutwell, J. M. Rock gypsum at Nephi, Utah. U. S. G. S. Bull. 
225. pp. 483-487. 1904. 

Burchard, E. F. Gypsum deposits in Eagle Co., Colo. U. S. G. S. 
Bull. 470. pp. 354-366, 1911. 

. Gypsum. U. S. G. S. Mineral Resources of U. S. for 

1909, part II, pp. 639-645, 1911. 

Eckel, E. C. Gypsum and gypsum products. U. S. G. S. Mineral 
Resources of the U. S. for 1905, pp. 1105-1115, 1906. 

Harder, E. C. The gypsum deposits of the Palen Mts., Riverside 
Co., Cal. U. S. G. S. Bull. 430, PP . 407-416, 1910. 

Hess, F. L. A reconnaissance of the gypsum deposits of California. 
U. S. G. S. Bull. 413, 37 pp. 1910. 

. Gypsum deposits near Cane Springs, Kern Co., Calif. 

U. S. G. S. Bull. 430, pp. 417-418, 1910. 

Kay, G. F. A new gypsum deposit in Iowa. U. S. G. S. Bull 580 
E. pp. 59-64. 1914. 

Lipton, C. T. Gypsum along the west flank of the San Rafael 
Swell, Utah. U. S. G. S. Bull. 530, pp. 221-231, 1913. 

Orton, Edward. Gypsum or land plaster in Ohio. U. S. G. S. 

Mineral Resources of the U. S. for 1887, pp. 596-601, 1888. 
Parker, E. W. Gypsum. U. S. G. S. 21st Ann. Rept. part 6 
(cont.) pp. 523-530, 1901. 

Shaler, M. K. Gypsum in Northwestern New Mexico. U. S. G. S. 
Bull, 315, pp. 260-265, 1907. 

Stone, R. W. Gypsum industry in 1912 and 1913. U. S. G. S. 
Mineral Resources of U. S. for 1912 and 1913; 1913 and 1914. 

. Gypsum, the mineral industry during 1912, pp. 440- 

450, 1913. 

Blake, W. P. Gypsum in Arizona. American Geologist, XVIII, 
p. 394, 1896. 





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GYPSUM 

By Frank L. Culin, Jr. 

The gypsum industry is a growing one, and will apparently con- 
tinue to grow for some time. Producers report better trade condi¬ 
tions for 1913 than for 1912. In ten years the production has 
jumped from 1,000,000 tons of crude gypsum in 1903 to more than 
2,500,000 tons in 1913. 

Gypsum was produced in eighteen states and in Alaska. Eighty- 
two quarries or mines were worked, while the total number of mills 
reported in 1913 w T as 67. New York was the largest producer of 
raw gypsum; Iowa ranked second and Michigan third. 

Gypsum finds its chief use in the manufacture of various plasters, 
such as plaster of Paris, molding and casting plaster, stucco, so-called 
“cement” plaster or hard w T all plaster, flooring plaster, hard finish 
plaster, etc. Refined grades of gypsum plaster are used in dental 
work, in the plate glass industry, and for making molds of various 
kinds, as for pottery, stereotypes, rubber stamps, etc. It is also used 
as a fertilizer, in paints, for interior decoration (alabaster) and for 
many other purposes (see Uses). 

Gypsum imported into the United States comes almost entirely 
from Nova Scotia and New Brunswick, entering through the ports 
of the North Atlantic states and New England. 

OCCURENCE. 

Gypsum occurs in three varieties: (1) Crystallized, or selenite; 
(2) fibrous, either coarse or fine, called satin spar; (3) massive, ala¬ 
baster and earthy or rock gypsum. 

The mineral anhydrite is also used in the gypsum industry. This 
mineral is anhydrous gypsum—that is, gypsum without the molecule 
of water. 

GYPSUM 

Composition : Gypsum is a hydrous calcium sulphate, (CaS0 4 -f- 
H 2 0) sulphur trioxide (SO s ) 46.6% lime (CaO) 32.5%, 
and w r ater (H 2 0) 20.9%. 

Form : Gypsum occurs in three forms: 

1. Crystallized, or selenite; colorless, transparent, in dis¬ 
tinct crystals or broad folia, somewhat flexible, and having a 
fibrous fracture. 

2. Fibrous, coarse or fine. Called satin-spar; when fine, 
it is fibrous, with pearly opalescence. 


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Arizona State Bureau of Mines 


3. Massive; alabaster, a fine grained variety, white or deli¬ 
cately shaded; earthy or rock gypsum, a dull colored rock, often 
impure with clay, calcium carbonate or silica. 

Gypsum crystallizes in the mono-clinic system, the crystals 
usually being simple, and commonly flattened. 

Color: Usually white, sometimes gray, reddish yellow, blue; im¬ 
pure varieties often black, brown, red, or reddish brown. 
Transparent to opaque. 

Lustre: In crystalline and fibrous forms, pearly and shining to sub- 
vitreous. Massive varieties often glistening, sometimes dull 
earthy. 

Hardness: Soft (1.5-2). 

Weight: Light. Sp. Gr. 2.314—2.328. 

Streak: White. 

Occurrence: Forms extensive beds in connection with various strati¬ 
fied rocks, especially limestones, and clay beds. It occurs occa¬ 
sionally in crystalline rocks. Is also a product of volcanoes, oc¬ 
curring where sulphur gases are escaping. It is produced also 
by the decomposition of pyrite when lime is present. When 
found in extensive beds, it has been precipitated from sea-waters. 

Blowpipe Tests: In the closed tube gives off water and becomes 
opaque. Fuses at 2.5-3 (readily fusible), coloring the flame 
reddish yellow. Ignited at a temperature not exceeding 26 de¬ 
grees C., it again combines with water when moistened, and 
becomes firmly solid. Soluble in HC1 (hydrochloric acid), and 
also in 400 to 500 parts of water. 

ANHYDRITE 

Composition: Anhydrous calcium sulphate, CaS0 4 . Sulphur tri¬ 
oxide (SO s ) 58.8% lime (CaO) 41.2%. 

Form: Crystallizes in the orthorhombic system. Crystals are not 
common. Usually massive, cleavable, fibrous, lamellar, and 
granular. 

Color: White, also grayish, bluish, and reddish. 

Streak: Grayish white. 

Hardness: Slightly harder than gypsum. (3-3.5) Brittle. 

Weight: Light. Sp. Gr. 2.899—2.985. 

Occurrence: In limestone strata, often with gypsum; very common 
in rock salt beds. 

Blowpipe Tests: Fuses rather easily (3), coloring the flame reddish 
yellow, and yielding an enamel like bead which reacts alkaline. 
On charcoal in reducing flame is reduced to a sulphide; with 


Mineral Technology Series No. 10 


5 


soda does not fuse to a clear globule, and is not absorbed by 
the coal like barite. It is decomposed, yielding a mass that 
blackens silver. Soluble in hydrochloric acid. 

USES OF GYPSUM 

The greater part of the gypsum produced in the United States 
and in foreign countries as well, is manufactured by grinding and 
by partial or complete calcination into the various plasters, such as 
plaster of Paris, molding and casting plaster, stucco, so-called “ce¬ 
ment” plaster or hardwall plaster, flooring plaster, hard finish plas¬ 
ter, etc. Refined grades of gypsum are used in dental work, in the 
plate glass industry, for making pottery molds, stereotype molds, 
molds for rubber stamps, and in various patent cements. The use 
of raw gypsum as a retarder in Portland cement is steadily increas¬ 
ing. Large quantities are ground without burning and used as land 
plaster or fertilizer. To a lesser degree gypsum is used in the manu¬ 
facture of paints, wall tints, crayon, paper, imitation meerschaum and 
ivory, and as an adulterant. The variety alabaster is much used for 
interior decoration. The variety selenite (the crystallized form) is 
in great demand for optical purposes, especially in the manufacture 
of scientific optical instruments. For this purpose the crystals must 
be very pure and free from flaws of any kind. 

A high grade of rock gypsum, in the pure or “neat” condition, is 
required for plaster of Paris and for dental molding and casting 
plasters. No foreign substance or retarder is used. These plasters 
are quick setting, and are white in color. 

Most of the so-called cement plaster is made from gypsite, an 
earthy, unconsolidated or sandy form of gypsum, with the addition of 
a suitable retarder. In many cases the foreign material contained 
in the gypsite is sufficient to take the place of any other retarder. If 
gypsite is ont to be had, these plasters are made from rock gypsum, 
by the addition of various minerals or organic retarders. 

A large part of the structural plaster now produced is gotten up 
in specially prepared forms for the convenience of the builder, as 
plaster board, solid and hollow blocks, and tiles. Plaster board is 
pressed from plaster interlaminated with thin sheets of cardboard 
felt or wood. It is furnished in various sizes and thicknesses. It is 
designed to be nailed directly to the studding, and to receive a coat 
of w T all plaster on its outer surface. Blocks and tiles are molded 
from fibred plaster, and are used for interior partitions and for roof¬ 
ing. This type has proved to be of value as a fire retarder. Gypsum 
tiles are lighter than clay tiles, are straight and true, can be cut with 


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Arizona State Bureau of Mines 


a hand saw and because of their lightness and size, can be laid very 
rapidly. Gypsum blocks have been used for exterior construction in 
some places, especially where the climate is arid, and has proven 
satisfactory. Gypsum has been extensively used for the exterior 
finish of temporary structures, such as exposition buildings. 

By far the most important single use of gypsum in this country 
is in the manufacture of plaster, especially wall plasters. Hard wall 
plaster consists of plaster of Paris, a fibre, as hair or wood fibre, and 
a retarder. It is of two general grades, one having a brown or gray 
coat, and the other a white or tinted finish coat. It is a little more 
expensive than lime, and not so good as lime for deadening sound, 
but is more convenient to handle than lump lime, and, due to its 
rapid setting, permits a job to be completed in less time. 

Under the name of “Keene’s” cement, a number of hard-finish 
anhydrous plasters are made from gypsum. This is made by add¬ 
ing alum or borax to de-hydrated, or calcined gypsum. This makes a 
very white and very hard plaster. It is used both as a wall and 
flooring plaster. 

Gypsum is also used in the manufacture of calcimines, water 
paints and tints, and as an ingredient in dry colors. When used 
in excess in mixed paints, it is regarded as an adulterant, but it 
may be used to a certain extent with oil paints, because it is chemically 
inactive, provided it is in unburned or dead-burned form. 

Some claim that the strength of gypsum plasters decrease with 
time. This may be true, or may not; at any rate the decrease, if 
any, is so very slight that it is hardly appreciable. 

METHODS OF PREPARATION 

In preparing the gypsum for market the stone is first broken in a 
crusher to about half or three-quarter inch size. It is then ground 
to a proper degree of fineness. If it is to be used for stucco it is 
calcined after being ground. In preparing some of the patent plasters, 
various adulterants and retarders are used. It is usually packed in 
bags or barrels. 

THE GYPSUM INDUSTRY 

In the United States there are 18 states producing gypsum, as 
well as Alaska. In 1913 there was a production of 2,599,508 tons 
of raw gypsum, as against 2,500,757 tons in 1912, an increase of 
98,751 tons. Gypsum sold for use in Portland cement and paint, 
and as land plaster amounted to 463,136 tons, valued at $697,066. 
The total value of gypsum and gypsum products produced in 1913 


Mineral Technology Series No. 10 


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was $6,774,822, as compared with $6,563,980 in 1912, an increase of 
$210,914. In 1914 the tonnage of gypsum produced was less than 
in 1913, but due to the advance in price of calcined gypsum the 
value of the product increased. 2,476,465 tons were mined in 1914, 
a decrease of 123,043 tons from 1913. The total value, however, 
was $6,895,989, an -increase of $121,167 over 1913. The states 
producing, gypsum and their rank remained unchanged during 1914. 

New work was the largest producer, followed by Iowa. It is 
also produced in the following states: Ohio, Texas, Kansas, Nevada, 
Oklahoma, Wyoming, Arizona, Colorado, Montana, New Mexico, 
South Dakota, Utah, Virginia, Oregon, and Alaska. Sales of gyp¬ 
sum products are credited to Illinois, Minnesota, Washington, and 
Wisconsin, although these states are not producers. 

Practically all of the gypsum imported into the United States 
comes from Nova Scotia, and New Brunswick. There was an in- 
crease in both quantity and value of imports in 1913—the total im¬ 
portation of unground gypsum in 1913 being 447,383 tons, valued 
at $473,594. The quantity of ground or calcined gypsum imported 
is very small. 

In foreign countries, France is the largest producer of gypsum, . 
and ranks next to the United States in the world’s production; the 
value of gypsum produced in France in 1911 being well over 
$3,000,000. Gypsum is also produced in Algeria, Australia, Canada, 
Cyprus, Bavaria, Greece, India, and the United Kingdom. 

ARIZONA DEPOSITS 

Gypsum occurs at several localities in Arizona, the following being 
noteworthy: Navajo County, Fort Apache Reservation, Snowflake, 
Winslow, and Woodruff; Cochise and Pinal Counties, along the 
San Pedro River, and at Douglas; Pima County, in the foothills of 
the Santa Catalina Mountains north of Tucson, and in the Santa Rita 
Mountains southeast of Tucson. The gypsum deposits in the Santa 
Rita Mountains are of considerable thickness and extent. The oc¬ 
currence on Fort Apache Reservation consists of large selenite crys¬ 
tals. It has been quarried at Douglas since 1908 and at Winslow 
since 1909. 

Another deposit has lately been discovered near Winkelman, Pinal 
County. U. S. Geological Survey men say that this deposit is the 
largest and most extensive deposit of high grade gypsum in the United 
States. This deposit will probably be worked in the near future. 


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Arizona State Bureau of Mines 


GEOLOGICAL CONDITIONS AND RELATIONS 

Gypsum occurs most frequently in sedimentary rocks, interbedded 
with shales, sandstones and limestones, and often closely associated 
with rock salt. It is also found as surface deposits mixed with clay 
(gypsite) or in the form of sand. It is often found in limited quan¬ 
tities in volcanic regions, especially in lavas. 

Practically all bedded deposits of gypsum have been formed by 
the evaporation of salt waters, either in inland seas, or in arms of 
the ocean. Gypsum may also be formed by the decomposition of 
sulphides, as pyrite, and the action of the sulphuric acid thus liberated 
on lime rocks. In volcanic regions, gypsum is formed by the action 
of sulphuric vapors on the lime of volcanic rocks. 

PRICES AND ECONOMIC CONSIDERATIONS 

The average proce of land plaster per ton at the mill was $1.75 
in 1913, a lower price than the same material brought in the three 
years previous. The average price of gypsum sold for the manufac¬ 
ture of paint, for Portland cement, for bedding plate glass, and for 
other purposes, was $1.47 per ton in 1913, as compared with $1.33 in 
1912. The average price of calcined gypsum products, including 
wall plaster, plaster of Paris, Keene’s cement, and dental plaster, was 
$3.43 per ton in 1913, the same as in 1912. The average price of 
unground, imported gypsum was $1,058 per ton in 1913. 

The commercial value of gypsum depends largely upon its ac¬ 
cessibility to the market, and its purity. These two features, of 
course, play a very important part in the development of any mineral 
industry and must be carefully considered. 

FUTURE OF THE INDUSTRY 

An investigation of some of the figures on production and price 
of gypsum in the past ten years show that the industry has had a very 
substantial growth. With the increasing use of gypsum in wall 
plasters, Keene’s cement, as a fertilizer, and in various other forms, a 
future growth seems well assured. 

In Arizona, however, the industry cannot be expected to assume 
much importance until transportation facilities are improved, nor 
until the population has increased to such an extent that a strong 
local demand for gypsum products will be created. 


